1. Technical Field
This is related to systems for real-time monitoring of position and alignment of components, such as in an engine or other machinery system.
2. Related Technology
Many industrial facilities and machines require knowledge of the precise relative location of different components. For example, particle accelerators and other systems used for high-energy physics research rely on knowledge of the precise relative positions of the many sensors, called tracking detectors, which sense the passage of charged particles generated by the high energy collider.
As an example, one of the components of Fermi National Accelerator Laboratory's Collider Detector at Fermilab (CDF) is a silicon detector, which includes seven concentric cylinders of silicon surrounding the beam pipe. Particles can pass through as many as all seven layers of silicon, leaving a trail of ions and electrons in each layer. This trail is recorded as a “hit” on that particular layer of silicon. By connecting the dots, scientists can determine the path of the particle. Because the silicon detector is located within a magnetic field, charged particles (such as electrons, muons, and charged hadrons) are forced to curve in their paths. The slower or less massive they are, the greater the magnet's effect on them, and the more they curve. The amount of curvature in the particle's track is used to determine the particle's mass and velocity, which provides valuable information about the particles that were produced immediately after a proton and antiproton collided. In these and other high energy physics experiments, precise track reconstruction is needed, so the location of each detector components must be known to high accuracy.
The original, or “three element,” RASNIK (Relative Alignment System of NIKHEF) system was developed at NIKHEF (Dutch National Institute of Subatomic Physics) in the 1980s and early 1990s. This system provided low-cost, real-time relative alignment monitoring of detector components for experimental high-energy physics experiments. The potential of the three-element RASNIK was limited, however, by the number of required independent elements and requirements on their relative spacing.
The three-element RASNIK system, shown in FIG. 1, includes three distinct elements. The first element is a diffuse light source 12 arranged to illuminate a coded mask 11. The coded mask 11 is a complex pattern imprinted on a thin substrate, such as quartz glass. The second element is a lens 14, which is mounted approximately midway between the projector 10 and the third element, a camera 16. The camera 16 could be, for example, a charge coupled device (CCD) grayscale micro-video camera. Each of the three elements is rigidly mounted to a physically independent object or structure. If any of the three elements moves, there is a corresponding movement of the projected image on the camera, allowing the system to track the relative alignment of the structures to which the elements are attached. The mask 11 can be a grid of black and white squares on the order of about 100 microns, photo-etched onto a thin quartz slide. Deviations from a perfect checkerboard grid are coded into the pattern to remove large scale ambiguities. The large number of black/white transitions in the pattern provides great statistical power for measuring relative movement.
D. Goldstein and D. Salzberg, “The RASNIK Real-Time Relative Alignment Monitor for the CDF Inner Tracking Detectors”, Nuclear Instruments and Methods in Physics Research A, Vol. 506, pp. 92-100, (2003), which is incorporated by reference herein in its entirety, describes the conception and installation of a “two-element” RASNIK in the Collider Detector at Fermilab (CDF) experiment's silicon tracking detector. This two-element system eliminated much of the ambiguity inherent in the original RASNIK system data, and increased the number of potential usage scenarios. FIG. 2 illustrates a two-element RASNIK system, in which the light source 22, coded mask 21, and lens 24 are integrated into a single projector unit 20. The second element is the camera 26, which is mounted on a structure that is physically independent from the first structure. The asymmetry between the distance D1 between the mask and the lens and the distance D2 between the lens and the camera magnifies the mask pattern, so masks with smaller dimensions are suitable. For example, the smallest squares on the mask can be approximately 20 microns.